The most common and most likely answer to this crossword clue is the 10 letter word CATAPHRACT
I've seen this crossword clue in The Times
C
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The straight part
We know that "Suit offering protection" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case suit offering protection = cataphract.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CATAPHRACT mean?
CATAPHRACT noun- Defensive armour covering the entire body of a soldier and often the soldier's horse as well, especially the linked mail or scale armour of some eastern nations.
- (by extension) An outer covering of some fish resembling armour or plate.
- Of a galley such as a trireme: with the upper tier of rowers shielded rather than exposed.